A Pitot Pressure Greater than Static Pressure?

2/14/2025

I have a project for which someone else did the flow test for me. At the test hydrant, I have a static of 69 psi and a residual of 64 psi at an elevation of 238'.

According to the information I was given at the flow hydrant, I have a static of 24 psi and a pitot of 25 psi at an elevation of 308'. With a coefficient of .90, they show a flow of 839 gpm.

My question is - do you think it is acceptable to use a flow test showing a pitot reading higher than the static read at the same hydrant?

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19 Comments

Dan

2/14/2025 08:14:08 am

Static at the flow hydrant should be identical to static at the test hydrant minus the elevation difference. A 70 ft elevation difference would yield a pressure delta of 30.31 psi. Your static at the higher elevation should be 38.69 psi. This is possibly a typo and I would ask for clarification from the testing party, or conduct a retest.

Patrick Spillane link

2/14/2025 08:16:08 am

It does seem a little odd. It would depend on what they are using as the "Nozzle" to measure the flow. I would confirm with them the device using for the measurement.

If they are using a Hose Monster, or other device with a known coefficeincy or K factor, then the hydrant butts coefficiency is irrelevant.

You can always contact Hose Monster for code and tech support.

Jesse

2/14/2025 08:16:13 am

Yep that's not a problem. The pitot measures or quantifies a different element of the water supply system. The static pressures measures the outward pressure the water provides while static or stationary. i.e. not moving.

Thje pitot pressure measures the dynamic pressure, or the velocity of the water.

The residual measures the outward pressure against the pipe when flowing.

So basically three different measurements all taken together to form a snapshot of the water supply characteristics at that moment in time.

Matt C

2/14/2025 08:17:36 am

Pressure and Velocity pressure aren't the same thing. If the idea of the residual number being higher bothers you then just express it in gpm rather than psi pitot. It's also easier for the layman to understand flow in gpm rather than wrap their head around two units representing two different things that look similar on paper.

Randy Kimbro

2/14/2025 08:18:20 am

I agree with Dan. Not only that, but the pitot reading should never be higher than the static reading. Since water is flowing with the pitot reading, that reading is theoretically impossible. As an AHJ, I would require a retest with us witnessing the process.

Leighton

2/14/2025 11:07:16 am

I bet the water supply has a pump that kicks on while a flow condition is happening. Or some other type of system in place.

Joe M

2/14/2025 08:21:54 am

Additional I would like to know if liquid filled gauges were use and if there were calibrated within the last year.

Is this a private or public main?

I assume no fire pump but if this is a public main a Water Department pump may have turned on., but not likely with those numbers.

Was the test for a sprinkle design or fire flow? If fire flow, I would use the 4-inch outlet or 2 -2 1/2 outlets to get a higher flow rate

Glenn Berger

2/14/2025 08:25:38 am

As others have said - Static pressure and pitot pressure measurements at the same hydrant are uniquely different. No issues with using those readings.

I would however review historically data ONLY to verify if the readings are consistent from previous tests.

Casey Milhorn

2/14/2025 08:37:11 am

Agree with most everyone else. Not an issue. I will add, too many people will try and use a residual pressure reading and equate that to a pitot reading. Though sometimes similar readings, it is NOT the same. Always get an actual pitot pressure to determine flow.

Also, I have had instances where my residual actually ticked up just a hair during flow tests when pumping stations are involved. The station recognized a large flow and loss of pressure and kicked on.

Lastly, that is a large difference from the static at the gauge hydrant to the static at the flow hydrant. Wouldn't hurt to confirm the elevation difference using google earth.

Franck

2/14/2025 08:47:30 am

Static and residual pressures are information related to the fire mains. They shouldn not be taken at the hydrant that is flowing.
The pitot pressure is not related to residual pressure. It is a measurement of the flow.
In your case, the flow is quite high (more than 800 gpm), so it is quite logical to have a high pitot pressure (for the flow measurement).
It is only surprising to have a measure of pressure at the flowing hydrant as this one is normally only use for flowing water and in no case to measure a pressure. Was it really static (i.e. before flowing) or some kind of mix between residual and the passing flow if pressure was taken while flowing?

I know that a common mistake is to use only one hydrant to make all measures.
First static. And then residual at the same time as the flow measurement. But this does not work, and the "residual" pressure obtained is generally much lower than the actual residual pressure because of the dynamic flow inside the hydrant.
Correct procedure is to flow one hydrant (and measure flow with pitot tube) and use a sprinkler riser or another hydrant to measure static and residual pressure.

Jose Figueroa

2/14/2025 08:49:34 am

Velocity pressure (Pitot Reading) is independent of static pressure. I cannot observe the diameter of the nozzle, and I find no issues with the test results. Thank you.

Jack G

2/14/2025 10:00:57 am

It sounds like the following might be affecting your test.
1. Improper alignment of the pitot in the stream or distance issues from the outlet.
2. Possible blockage in the supply down to the second hydrant from sediment.
3. Possible closed hydrant valve, always count your turns.
4. Possible pitot clogged from prior test at higher outlet.
5. Air trapped in the system,
6. Low pressure in the main .
7. Should have tried a stream straightener on the outlet.
8. Check for clogs or damage in the gauges or pitot. ( I remove the gauges and blow compressed air into the devises )
I m guessing #5 and #6.

Robert Hughes

2/14/2025 10:20:46 am

The guages used are not micrometers, and are prone to vibration.
1 psi differential is more accuracy than most guages provide.

Pete H

2/14/2025 10:38:25 am

Seems fine.... any chance you can measure flow at the hydrant in GPM and see if it is in line with the Pitot reading at that static?

Dave L.

2/14/2025 10:39:03 am

I can't overstate (so I'm going to repeat it) what was mentioned above about making sure the gauges have been calibrated within the last year. Ours go in same time every year (just took mine in yesterday) and it is not unusual for one to need calibration, and last year one was found to be unrepairable. (And if I'm going to put my name on it, the report would be more presentable.)

Jerry Clark

2/14/2025 11:59:16 am

No, it is not acceptable to use a flow test where the pitot reading is higher than the static pressure at the same hydrant. This suggests an error in measurement or recording.

The pitot gauge measures the velocity pressure of the flowing water. The static pressure represents the pressure in the system before flow occurs. If the pitot pressure is higher than the static pressure at the same hydrant, it would indicate that the velocity pressure is exceeding the available system pressure, which is physically impossible.

I would hazard that the static pressure at the flow hydrant may have been misread or the pitot reading may not have been taken correctly. It could also be if the 24 psi static was actually measured at a different location, it may not reflect the true pressure at the flow hydrant.

I've also had issues before if the hydrant or nozzle was not fully opened or if there were obstructions, it could lead to misleading pitot readings.

I would have them verify the current data, or just have them perform the test over and recalculate. In short, the test results as reported do not appear reliable, and I would recommend a re-evaluation before using this data for hydraulic analysis.

Ivan J. Humberson

2/17/2025 11:28:13 am

As some others have noted, it very well may be the gauges were not accurate. Look at line #11 - the test hydrant is noted to be at 238' elevation, and the Flow hydrant at 308' (70' higher). That 70' elevation difference should work out to be a 30 psi lower static pressure at the flow hydrant, compared to the static pressure at the test hydrant. The static reading (24 psi) is 45 psi lower than the static at the test hydrant. Something is amiss - I would have the contractor re-do the flow test and, if possible, witness the flow test.

Jerry Graupman

2/19/2025 10:50:06 am

Pitot pressure can NEVER be higher than static pressure.

If it is, something was measured incorrectly or a gauge is wrong.

An alternate term for pitot pressure is "total" pressure, which means it's the sum of dynamic pressure and static pressure. Total pressure is always the same, it only changes with how fast a fluid is moving, unless you add energy to it with a pump or fan.

Since there's no addition of energy when it goes from inside to outside the hydrant, the total pressure cannot go up. What's happening when water exits the hydrant is energy is being shifted completely from static to dynamic.. Since there is no vessel to hold it in, static pressure goes to zero and everything is converted to dynamic pressure (with some friction loss)

Freddie Bell link

2/20/2025 12:42:11 am

Sometimes, errors with readings and calculations are made. There are many potential causes.

Recently, I received a fire hydrant flow test indicating 2.5" outlet #1 was 34 psi and outlet #2 was 24 psi.

In my case, paperwork reviews are enlightening.

A Pitot Pressure Greater than Static Pressure?

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